In a spark plug for igniting combustion gas in an internal combustion engine, a gap for discharging a spark is formed between a center electrode and a ground electrode. Here, a spark plug is known in which a noble metal tip is mounted, via an intermediate member, to an electrode base material of the ground electrode (e.g., Japanese Patent Application Laid-Open (kokai) No. 2013-33670). The intermediate member is used for reducing a possibility of occurrence of a trouble when a noble metal tip is directly mounted on an electrode base material. For example, the amount of use of a noble metal tip can be reduced by interposing the intermediate member.
In the technique of Japanese Patent Application Laid-Open (kokai) No. 2013-33670, joining strength between the electrode base material and the intermediate member is improved by defining, when the intermediate member is joined to the electrode base material by welding, a relationship among a dimension of a nugget formed between the intermediate member and the electrode base material, a height from an arrangement surface of the electrode base material to an end surface of the noble metal tip, and the maximum width of the noble metal tip.
Incidentally, the diameter of a noble metal tip needs to be increased from the viewpoint of improvement in wear resistance. In the case where the diameter of a noble metal tip is increased, when the noble metal tip and the intermediate member are joined to each other by laser welding, stress applied to a melt portion formed between the noble metal tip and the intermediate member is likely to be large. Thus, it may be difficult to ensure joining strength between the noble metal tip and the intermediate member. Therefore, a technique is desired which allows improvement of not only joining strength between the electrode base material and the intermediate member but also joining strength between the noble metal tip and the intermediate member.
The present specification discloses a technique that allows improvement of joining strength between the noble metal tip and the intermediate member while wear resistance of a spark plug is improved.